This section provides background information related to the present disclosure which is not necessarily prior art.
As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching an injection molded PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of approximately 250° F.-350° F. (approximately 121° C.-177° C.), and holding the blown container against the heated mold for approximately two (2) to five (5) seconds. Manufacturers of PET juice bottles, which must be hot-filled at approximately 185° F. (85° C.), currently use heat setting to produce PET bottles having an overall crystallinity in the range of approximately 25%-35%.
Unfortunately, PET is a poor barrier to oxygen. One of the main factors that limit the shelf life of foods and beverages (herein known as “fills”) in PET containers is the ingress of oxygen through the walls of the container followed by oxidation of the fill. Many strategies have been employed to reduce the amount of oxygen in contact with food in PET containers. Some strategies include using package barrier coatings, such as chemical vapor deposited (CVD) aluminum oxide or silicon oxide. Still further, some strategies include the use of PET barrier additives that create physical barriers to oxygen diffusion through the packaging (e.g., nylon, nanoclays). However, these barrier additives may include a monolayer barrier blend that is incorporated into the entire preform resulting in scrap material, such as from a removed dome or moil portion, having high levels of barrier material. This scrap material having high levels of barrier material is incapable of being reused effectively in the plant to manufacture subsequent containers. That is, the high levels of barrier material in the strap material is generally non-conducive to in-plant recycling, thereby leading to excessive material waste and increased manufacturing costs.
In some applications, embedded barrier layers have been incorporated in a multilayer construction of the container to overcome penetration of oxygen into the container. However, such embedded barrier layers can often delaminate if the container is trimmed or otherwise cut improperly. That is, in some cases of container manufacturer, additional portions of the container are created that must be removed prior to final construction along a cut interface. These additional portions may include a dome section and/or moil portion. In some cases, manufacturers have only continued the embedded barrier layer to a position below the intended cut interface, thereby preventing the cut interface from exposing the laminated, multilayer configuration. With this option, it is difficult to control the barrier layer to ensure adequate barrier coverage (that is, that the embedded barrier layer does not stop too short from the finish of the container thereby exposing the contents to oxygen) while also preventing delamination caused by trimming through the multiple layers. In some applications, attempts have been made to heat and curl the finish of the container after trimming to prevent delamination. However, such technique adds additional manufacturing steps and required equipment, thereby increasing costs and time.